1. Field of the Invention
This invention relates to a method and a system for calibrating a hands-free system, such as a hands-free system utilized in a vehicle.
2. Related Art
In vehicles, hands-free systems are used for outputting audio signals of a remote subscriber using the loudspeakers that are normally provided in a vehicle. Several microphones are installed in different locations in the vehicle for picking up the signals of the passengers in the vehicle. In connection with a hands-free system, such microphones can be used instead of the microphones provided in the mobile phone. Additionally, signal processing can be used which is adapted to the vehicle environment. By way of example, noise reduction systems are used that are optimized to the different types of vehicles and that reduce the noise component in the signal detected by the microphone of the hands-free system.
For improving the speech quality in hands-free systems, an adaptation of the hands-free system to the mobile phone that is used in connection with the hands-free system is necessary. By way of example, a signal level coming from a remote user should always result in a same signal level that is output via the loudspeaker. The signal level should not depend on the mobile phone used in connection with the hands-free system. Mobile phones normally have different signal output levels so that an equalization of the signal level depends on the used mobile phone.
In the art, it is known to provide equalization parameters for the most commonly used mobile phones in order to adapt the hands-free system to the used mobile phones. These parameters can be stored in a storage unit of the hands-free system. When a mobile phone is connected to a hands-free system either wireless via Bluetooth or when the mobile phone is put into a cradle, the mobile phone is identified by the type of mobile phone connected. When it is known which type of mobile phone is being utilized, the corresponding calibration or equalization parameters can be loaded and used in a calibration unit to adapt the hands-free system to the mobile phone (e.g. by adapting the frequency response or by adapting the signal level).
This kind of calibration, however, has several drawbacks. When the calibration parameters are only determined for a specific type of mobile phone, an individual calibration to the mobile phone is not possible. There exist significant differences between different mobile phones of a certain type. Such differences may be due to the amplifiers used in the mobile phones. These amplifiers often have a varying output signal level of ±3 to 6 dB. An adaptation to the type of mobile phone used does not result in an optimum speech signal quality, as the processing parameters vary within one type of mobile phone.
Furthermore, it is possible that the calibrating parameters of a mobile phone change. By way of example, when a new software release is used in the mobile phone, the signal processing changes. In the event that a predetermined set of calibration parameters is used, these calibration parameters will be continuously used after the software update even if the characteristics of the mobile phone have changed. Even if new calibrated parameters are transmitted together with the software update, the problem may exist that some of these mobile phones use an older software version than other mobile phones of the same type. Normally, a differentiation between the different software types is not possible, as the actual version of the used software is normally not transmitted to a hands-free system during identification of the mobile phone.
A further disadvantage of known systems lies in the fact that calibration parameters can only be determined for mobile phones already existing on the market. When a new mobile phone is developed, the hands-free system may not have the corresponding calibration parameters, as the life cycle of a mobile phone is normally much shorter than the life cycle of a hands-free system used in a vehicle.
In summary, in prior art hands-free systems several drawbacks exist resulting in a non-optimal speech quality of the hands-free system. Accordingly, a need exists to provide a hands-free system in which the speech quality can be further improved and in which each individual mobile phone can be adjusted or calibrated to the hands-free system.